Numerous systems are known to provide such sealing.
U.S. Pat. No. 2,379,070 discloses a system which comprises a tubular primary casing section which in use is located in the primary borehole in the region of the branchpoint and a branch casing section which is welded to the primary casing section and which in use is located in the branch borehole.
In this known system the primary casing section is a tubular having a curved lower end which forms a lip which can be pulled against an existing casing inside the primary borehole by pulling or pushing the branch casing section into the branch borehole. As a result the tubular wall of the primary casing section is pulled against the wall of the existing casing to provide a seal that covers the window that has been cut into the existing casing at the location where the branch borehole intersects the primary borehole.
Soviet Union patent specification SU-663825 discloses another sealing system where a branch casing is welded to a primary casing section that is formed by a corrugated sleeve which is expanded downhole by an expansion mandrel.
International patent application WO 9403699 discloses the use of a primary casing section that is made of a memory alloy which is expanded by applying heat to provide a fluid-tight juncture.
A disadvantage of the known sealing systems is that they require the use of a primary casing section which is complex and expensive to make, that they require complex installation procedures and that the juncture seal provided can be prone to leakage.
It is an object of the present invention to provide a system for sealing the intersection between a primary and branch borehole which can be made and installed in a cost effective manner and which provides an effective seal.